mojo: Add proper implementation of DetachFromClient to OutputSurfaceMojo.

mojo/cc/output_surface_mojo.cc

Index: mojo/cc/output_surface_mojo.cc
diff --git a/mojo/cc/output_surface_mojo.cc b/mojo/cc/output_surface_mojo.cc
index c0998cae543c5e140d719e4a770d57f1cad8f95d..f06835f43c49a95960ce017900f6f44137e5fd00 100644
--- a/mojo/cc/output_surface_mojo.cc
+++ b/mojo/cc/output_surface_mojo.cc
@@ -30,6 +30,11 @@ bool OutputSurfaceMojo::BindToClient(cc::OutputSurfaceClient* client) {
   return cc::OutputSurface::BindToClient(client);
 }
 
+void OutputSurfaceMojo::DetachFromClient() {
+  surface_.reset();
+  cc::OutputSurface::DetachFromClient();
+}
+
 void OutputSurfaceMojo::SwapBuffers(cc::CompositorFrame* frame) {
   // TODO(fsamuel, rjkroege): We should probably throttle compositor frames.
   client_->DidSwapBuffers();